A cohort study of 55,371 menopausal American women has found no significant differences among different ethnic groups for the increased risk of breast cancer related to hormone replacement therapy. The study, published online September 16, 2005 in the International Journal of Cancer, the official journal of the International Union Against Cancer (UICC), did find that leaner women taking hormone replacement therapy had a relatively greater increase in breast cancer risks than heavier women. The study is available via Wiley InterScience (www.interscience.wiley.com/journal/ijc).

Previous studies have already revealed a link between menopausal hormone therapy and increased breast cancer risk. The relative risk is greatest for women taking estrogen-progestin therapy. But most studies have focused on white women, and few considered prognostic factors such as body mass index, stage of disease, histologic subtype, and hormone receptor status. Researchers, led by Sulggi Lee of the Keck School of Medicine at the University of Southern California, Norris Comprehensive Cancer Center decided to examine these factors in relation to the increased breast cancer risk associated with hormone therapies.

They utilized the Multiethnic Cohort Study which includes 215,251 men and women, aged 45 to 75 who were living in Hawaii or California in 1993. They focused on 55,371 African-American, Native Hawaiian, Japanese American, Latina and White postmenopausal women and gathered health and medical information each woman had provided upon her enrollment into the cohort. They then crosschecked for subsequent incidents of invasive breast cancer using local and state cancer registries through the end of 2001. There were 1,615 such cases, and for each, the researchers obtained data on tumor characteristics, cancer stage and receptor status.

The researchers found that current use of estrogen-progestin therapy was associated with an increased breast cancer risk--29 percent higher after five years of use. The association applied to women in all ethnic groups. Current use of estrogen therapy was also associated with increased risk--10 percent higher after five years of use. This association was seen in all ethnic groups except for African-Americans.

Interestingly, relative breast cancer risks for current use of estrogen-progestin therapy was greater for women with low body mass index (BMI below 25), even though clear increases were also seen in women with higher BMIs. "Data on this aspect of the relationship between hormone therapy use and risk is scarce and it is too early to draw a firm conclusion," the authors report.

The effect of current estrogen-progestin therapy use on localized disease was greater than on advanced disease, in agreement with most previous studies. And hormone therapy was associated with increased risk of both ductal and lobular carcinomas. Estrogen-progestin therapy was significantly associated with ER+/PR+ tumors, but non-significantly associated with increases in ER+/PR- and ER-/PR+ tumors. Estrogen therapy was associated with both ER+/PR+ and ER+/PR- tumors--all consistent with the findings of earlier studies.

In interpreting results of this study, "it is important to consider certain methodological issues," say the authors. It is possible that patients were lost to follow-up, though the out-migration from California and Hawaii is very low. It's also possible that results were biased upward because users of hormone therapy are screened more frequently for breast cancer; however, adjustment of the results for the frequency of mammograms reported by the women at baseline had very little effect on these results. Lastly, the true duration of hormone therapy is unknown, since use was only assessed at baseline; however, the results quoted above are minimum estimates of risk as they were based on assuming that women using hormones at baseline continued to do so.

In conclusion, report the authors, "this study provides some of the first results comparing breast cancer risk among different racial/ethnic groups in relation to hormone therapy use. The results suggest that risk among women of other races is similar to the risk previously reported among whites."

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